ACPI: Make seemingly useless check in osl.c more understandable

There is a seemingly useless check in drivers/acpi/osl.c added by
commit bc73675 (ACPI: fixes a false alarm from lockdep), which really
is necessary to avoid false positive lockdep complaints.  Document
this and rearrange the code related to it so that it makes fewer
checks.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>

drivers/acpi/osl.c

@@ -932,7 +932,7 @@ static acpi_status __acpi_os_execute(acpi_execute_type type,
 	 * having a static work_struct.
 	 */
 
-	dpc = kmalloc(sizeof(struct acpi_os_dpc), GFP_ATOMIC);
+	dpc = kzalloc(sizeof(struct acpi_os_dpc), GFP_ATOMIC);
 	if (!dpc)
 		return AE_NO_MEMORY;
 
@@ -944,17 +944,22 @@ static acpi_status __acpi_os_execute(acpi_execute_type type,
 	 * because the hotplug code may call driver .remove() functions,
 	 * which invoke flush_scheduled_work/acpi_os_wait_events_complete
 	 * to flush these workqueues.
+	 *
+	 * To prevent lockdep from complaining unnecessarily, make sure that
+	 * there is a different static lockdep key for each workqueue by using
+	 * INIT_WORK() for each of them separately.
 	 */
-	queue = hp ? kacpi_hotplug_wq :
-		(type == OSL_NOTIFY_HANDLER ? kacpi_notify_wq : kacpid_wq);
-	dpc->wait = hp ? 1 : 0;
-
-	if (queue == kacpi_hotplug_wq)
+	if (hp) {
+		queue = kacpi_hotplug_wq;
+		dpc->wait = 1;
 		INIT_WORK(&dpc->work, acpi_os_execute_deferred);
-	else if (queue == kacpi_notify_wq)
+	} else if (type == OSL_NOTIFY_HANDLER) {
+		queue = kacpi_notify_wq;
 		INIT_WORK(&dpc->work, acpi_os_execute_deferred);
-	else
+	} else {
+		queue = kacpid_wq;
 		INIT_WORK(&dpc->work, acpi_os_execute_deferred);
+	}
 
 	/*
 	 * On some machines, a software-initiated SMI causes corruption unless